Esters of polyhydric alcohols, also known as polyol esters, find a variety of uses on a large scale in industry, for example as plasticizers or lubricants. The selection of suitable starting materials allows the physical properties, for example boiling point or viscosity, to be controlled, and the chemical properties, such as hydrolysis resistance or stability to oxidative degradation, to be taken into account. Polyol esters can also be tailored to the solution of specific performance problems. Detailed overviews of the use of polyol esters can be found, for example, in Ullmann's Encyclopaedia of Industrial Chemistry, 5th edition, 1985, VCH Verlagsgesellschaft, Vol. A1, pages 305-319; 1990, Vol. A15, pages 438-440, or in Kirk Othmer, Encyclopaedia of Chemical Technology, 3rd edition, John Wiley & Sons, 1978, Vol. 1, pages 778-787; 1981, Vol. 14, pages 496-498.
The use of polyol esters as lubricants is of great industrial significance, and they are used particularly for those fields of use in which mineral oil-based lubricants meet the requirements set only incompletely. Polyol esters are used especially as turbine engine and instrument oils. Polyol esters for lubricant applications are based frequently on 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 1,2-hexanediol, 1,6-hexanediol, neopentyl glycol, trimethylolpropane, pentaerythritol, 2,2,4-trimethylpentane-1,3-diol, glycerol or 3(4)-8(9)-dihydroxymethyltricyclo[5.2.1.02,6]-decane, also known as TCD alcohol DM, as the alcohol component.
Polyol esters are also used to a considerable degree as plasticizers. Plasticizers find a variety of uses in plastics, coating materials, sealing materials and rubber articles. They interact physically with high molecular weight thermoplastic substances, without reacting chemically, preferably by virtue of their swelling and dissolution capacity. This forms a homogeneous system, the thermoplastic range of which is shifted to lower temperatures compared to the original polymers, one result being that the mechanical properties thereof are optimized, for example deformation capacity, elasticity and strength are increased, and hardness is reduced.
In order to open up the widest possible fields of use to plasticizers, they must fulfil a series of criteria. They should ideally be odourless, colourless, and light-, cold- and heat-resistant. Moreover, it is expected that they are insensitive to water, comparatively non-flammable and not very volatile, and are not harmful to health. Furthermore, the production of the plasticizers should be simple and, in order to meet ecological requirements, avoid waste substances, such as by-products which cannot be utilized further and wastewaters comprising pollutants.
A specific class of polyol esters (they are referred to as G esters for short) contains diols or ether diols as the alcohol component, for example ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, 1,2-propylene glycol or higher propylene glycols. They can be prepared in different ways. In addition to the reaction of alcohol and acid, optionally in the presence of acidic catalysts, further processes are employed in practice to obtain G esters, including the reaction of diol with acid halide, the transesterification of a carboxylic ester with a diol, and the addition of ethylene oxide onto carboxylic acids (ethoxylation). In industrial manufacture, only the direct reaction of diol and carboxylic acid and the ethoxylation of carboxylic acids have become established as production processes, preference usually being given to the esterification of diol and acid. This is because this process can be performed with no particular complexity in conventional chemical apparatus, and it affords chemically homogeneous products. Compared to this, ethoxylation requires extensive and costly technical equipment. Ethylene oxide is a very reactive chemical substance. It can polymerize explosively and forms explosive mixtures with air within very wide mixing ranges. Ethylene oxide irritates the eyes and respiratory pathways, leads to chemical burns and to liver and kidney damage, and is carcinogenic. The handling thereof therefore entails extensive safety measures. Moreover, scrupulous cleanliness of storage apparatus and reaction apparatus has to be ensured, in order to rule out the formation of undesired impurities as a result of side reactions of the ethylene oxide with extraneous substances. Finally, the reaction with ethylene oxide is not very selective, since it leads to mixtures of compounds of different chain length.
The direct esterification of alcohols with carboxylic acids is one of the basic operations in organic chemistry. In order to increase the reaction rate, the conversion is typically performed in the presence of catalysts. The use of one of the reactants in excess and/or the removal of the water formed in the course of the reaction ensures that the equilibrium is shifted in accordance with the law of mass action to the side of the reaction product, i.e. of the ester, which means that high yields are achieved.
Comprehensive information regarding the preparation of esters of polyhydric alcohols, also including esters of ethylene glycols and fatty acids, and regarding the properties of selected representatives of these compound classes can be found in Goldsmith, Polyhydric Alcohol Esters of Fatty Acids, Chem. Rev. 33, 257 ff. (1943). For example, esters of diethylene glycol, of triethylene glycol and of polyethylene glycol are prepared at temperatures of 130 to 230° C. over reaction times of 2.5 to 8 hours. To remove the water of reaction, carbon dioxide is used. Suitable catalysts mentioned for the esterification of polyhydric alcohols are inorganic acids, acidic salts, organic sulphonic acids, acetyl chloride, metals or amphoteric metal oxides. The water of reaction is removed with the aid of an entraining agent, for example toluene or xylene, or by introducing inert gases such as carbon dioxide or nitrogen.
The production and the properties of fatty acid esters of the polyethylene glycols are discussed by Johnson (edit.), Fatty Acids in Industry (1989) Chapter 9, Polyoxyethylene Esters of Fatty Acids, and a series of preparative hints are given. Higher diester concentrations are achieved by the increase in the molar ratio of carboxylic acid to glycol. Suitable measures for removing the water of reaction are azeotropic distillation in the presence of a water-immiscible solvent, heating while passing through an inert gas, or performing the reaction under reduced pressure in the presence of a desiccant. When the addition of catalysts is dispensed with, longer reaction times and higher reaction temperatures are required. Both reaction conditions can be made milder by the use of catalysts. In addition to sulphuric acid, organic acids such as p-toluenesulphonic acid and cation exchangers of the polystyrene type are the preferred catalysts. The use of metal powders, such as tin or iron, is also described. According to the teaching from U.S. Pat. No. 2,628,249, colour problems in the case of catalysis with sulphuric acid or sulphonic acid can be alleviated when working in the presence of activated carbon.
Further metallic catalysts used to prepare polyol esters are also alkoxylates, carboxylates or chelates of titanium, zirconium or tin, for example according to U.S. Pat. No. 5,324,853 A1. Such metal catalysts can be considered as high-temperature catalysts, since they achieve their full activity only at high esterification temperatures, generally above 180° C. They are frequently added not at the start of the esterification reaction, but after the reaction mixture has already been heated up and has reacted partly with elimination of water. In spite of the relatively high reaction temperatures and relatively long reaction times required compared to the conventional sulphuric acid catalysis, crude esters with a comparatively low colour number are obtained in the case of catalysis with such metal compounds. Common esterification catalysts are, for example, tetra(isopropyl) orthotitanate, tetra(butyl) orthotitanate, tetra(butyl) zirconate or tin(II) 2-ethylhexanoate.
The catalytic esterification reaction of polyols with carboxylic acids achieves, based on the component present in deficiency, a high conversion within a comparatively short time, but a comparatively long reaction time has to be accepted for the remaining conversion to the desired polyol esters. Although a polyol ester is obtained with an acceptable residual content of partly esterified products, expressed by the hydroxyl number in mg KOH/g (to DIN 53240) or by the content of partly esterified products determined by gas chromatography, long reaction times are economically disadvantageous since they limit the performance of the industrial production plant. In order also to accelerate the residual conversion, U.S. Pat. No. 5,324,853 A1 proposes intensive mixing of the reaction mixture.
After the esterification reaction has ended, sufficient removal of the metal catalyst has to be ensured, since metal traces in the purified polyol esters can impair the use thereof as plasticizers or lubricants by, for example, influencing the electrical conductivity or the stability to atmospheric oxygen. According to the procedure from U.S. Pat. No. 5,324,853 A1, the crude esterification mixture is admixed with an aqueous sodium carbonate solution and optionally with activated carbon. This procedure hydrolyses the metal compounds to insoluble solids, which can be filtered off before the further workup of the crude ester compound.
According to U.S. Pat. No. 4,304,925 A1, the crude esterification product, before addition of alkali, is first admixed with water and treated under hot conditions. This converts the hydrolysed metal compounds to readily filterable precipitates.
EP 0 356 255 A2 discusses the esterification of propoxylated glycerol and saturated or unsaturated C10-C24 fatty acids in the presence of tin catalysts and titanate catalysts. The crude ester is treated with sodium hydroxide solution and then filtered in the presence of a filtration aid.
U.S. Pat. No. 4,526,725 A1 discloses chelated alkyl titanate catalysts and the use thereof as an esterification catalyst. The crude ester is subjected to a steam distillation to remove alcohol residues.
The prior art for preparation of polyol esters under metal catalysis requires either a special reactor design in order to complete the esterification reaction within an economically acceptable time, or an additional treatment with water under hot conditions, in order to substantially completely remove the metallic catalyst after the esterification reaction has ended with formation of hydrolysis products which can be filtered off readily.
It was therefore an object of the present invention to improve the known processes and to optimize the process by adjusting and simplifying the successive component steps of the overall process, and to simplify the production of polyol esters in high quality, such that polyol esters can have a maximum variety of applications.